Method of monitoring a magnetic field drift of a magnetic resonance imaging apparatus

ABSTRACT

The invention enables to monitor a magnetic field drift of a magnetic resonance imaging apparatus on the basis of the magnetic resonance signals, which are acquired during magnetic resonance image data acquisition, such as by single shot EPI or by a gradient echo sequence. The phases of at least two magnetic resonance signals are acquired an echo time after the corresponding RF excitations. This corresponds to the central k-space line, which has frequency encoding but no phase encoding. The difference of two consecutive phase measurements, which are acquired at a certain time interval provides the shift of the resonance frequency. This enables monitoring of the shift of the resonance frequency and compensation of the magnetic field drift.

FIELD OF THE INVENTION

The present invention is related to the field of magnetic resonanceimaging (MRI), and more particularly without limitation to magneticfield drift compensation.

BACKGROUND AND PRIOR ART

Prior art magnetic resonance imaging apparatus' generate a uniformmagnetic field which is also referred to as the main field or thepolarising field B₀. The purpose of the magnetic field is that theindividual magnetic moments of the spins in the tissue to be visualisedattempt to align with this polarising field, but precess about it inrandom order at a characteristic Larmor frequency which is determined bythe gyromagnetic constant of the spins and the polarising magnetic fieldB₀.

However during operation of a magnetic resonance apparatus the magneticfield can vary over time due to various reasons. For example when themagnetic resonance apparatus is under heavy duty the passive iron shimpieces of the coils which generate the magnetic field heat up whichleads to a fluctuation of the magnetic field.

U.S. Pat. No. 6,294,913b1 shows a method for compensation of variationsin the polarising magnetic field during magnetic resonance imaging.Monitor signals are acquired in an interleaved manner during a scan withthe MRI system. Frequency changes caused by variations in the polarisingmagnetic field B₀ are measured using the monitor signals, and thesemeasured frequency changes are employed to compensate image dataacquired during the scan. This compensation is achieved by changing thefrequency of the RF transmitter and receiver to offset the effects ofchanges in B₀. A disadvantage of this compensation method is thatspecial monitor signals are required for the compensation.

It is therefore an object of the present invention to provide for animproved method of monitoring and compensating a magnetic field drift.

SUMMARY OF THE INVENTION

The present invention provides for a method of monitoring a magneticfield drift of a magnetic resonance imaging apparatus which does notrequire special monitor signals or monitor sensors. Rather the method ofthe invention can be performed on the basis of the magnetic resonancesignals which are acquired during magnetic resonance image dataacquisition, such as by single shot EPI or by a gradient echo sequence.

In essence the phases of at least two magnetic resonance signals areacquired an echo time after the corresponding RF excitations. Thiscorresponds to the central k-space line which has frequency encoding butno phase encoding. This central k-space is usually designated ask_(y)=0. The difference of two consecutive phase measurements which areacquired at a certain time interval provides the shift of the resonancefrequency. This enables monitoring of the shift of the resonancefrequency and compensation of the magnetic field drift.

In accordance with a further preferred embodiment of the invention asingle shot EPI method is used for the magnetic resonance dataacquisition. Single shot EPI has the advantage that every dataacquisition contains the k_(y)=0 line such that for each dataacquisition the required phase information can be obtained.

In accordance with a further preferred embodiment of the invention agradient echo sequence is used for the magnetic resonance image dataacquisition. During such a gradient echo sequence the k-space is scannedalong an arbitrary trajectory which typically also contains the k-spaceline k_(y)=0. For example a full gradient echo sequence has 256 dataacquisitions one of which is representative of k_(y)=0. The dataacquisition for k_(y)=0 enables to obtain the required phase informationonce for each complete gradient echo sequence.

In accordance with a further preferred embodiment of the invention extrascans along k_(y)=0 are performed during a gradient echo sequence inorder to obtain multiple phases during the complete sequence. Thisenables to monitor a magnetic field drift which occurs during a singlegradient echo sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will bedescribed in greater detail by making reference to the drawings inwhich:

FIG. 1 shows a block diagram of a magnetic resonance apparatus withmagnetic field drift compensation,

FIG. 2 is illustrative of the phase determination for a single shot EPI,

FIG. 3 is illustrative of a flow chart for monitoring and compensatingthe magnetic field drift in the case of single shot EPI, and

FIG. 4 is illustrative of a flow chart for magnetic field monitoring andcompensation if a gradient echo sequence is employed.

DETAILED DESCRIPTION

FIG. 1 shows a magnetic resonance device 1 which includes a first magnetsystem 2 for generating a steady magnetic field, and also severalgradient coils 3 for generating additional magnetic fields having agradient in the X, Y, Z directions. The Z direction of the co-ordinatesystem shown corresponds to the direction of the steady magnetic fieldin the magnet system 2 by convention. The measuring co-ordinate systemx, y, z to be used can be chosen independently of the X, Y, Z systemshown in FIG. 3. The gradient coils are fed by a power supply unit 4. AnRF transmitter coil 5 serves to generate RF magnetic excitation pulsesand is connected to an RF transmitter and modulator 6.

A receiver coil is used to receive the magnetic resonance signalgenerated by the RF field in the object 7 to be examined, for example ahuman or animal body. This coil may be the same coil as the RFtransmitter coil 5. Furthermore, the magnet system 2 encloses anexamination space which is large enough to accommodate a part of thebody 7 to be examined. The RF coil 5 is arranged around or on the partof the body 7 to be examined in this examination space. The RFtransmitter coil 5 is connected to a signal amplifier and demodulationunit 10 via a transmission/reception circuit 9.

The control unit 11 controls the RF transmitter and modulator 6 and thepower supply unit 4 so as to generate special pulse sequences whichcontain RF pulses and gradients. The phase and amplitude obtained fromthe demodulation unit 10 are applied to a processing unit 12. Theprocessing unit 12 processes the presented signal values (also referredto as k-space) so as to form an image by transformation. This image canbe visualized, for example by means of a monitor 13.

Processing unit 12 determines a shift Δƒ of the resonance frequency onthe basis of the phase information obtained from the demodulation unitand provides the shift Δƒ to the control unit 11 for compensation of theshift.

The acquisition of the required phase information is explained in moredetail in the following by making reference to FIG. 2. First an RFmagnetic excitation pulse 200 is generated by the transmitter coil (cf.transmitter coil 5 of FIG. 1). Further a slice selecting gradient G_(Z)pulse 202, a bipolar switched gradient 204, and a phase encodinggradient 206 are applied. The application of these gradients 202, 204and 206 for single shot EPI magnetic resonance data acquisition is assuch known from the prior art.

The single shot EPI provides measurement signal 208. The peak ofmeasurement signal 208 is the echo time TE after the RF magneticexcitation pulse 200. The phase of measurement signal 208 at time TEwith respect to the RF magnetic excitation pulse 200 is determined forthe purpose of monitoring a drift of the magnetic field and acorresponding drift of the resonance frequency. In order to determinethe resonance frequency drift, if any, the phase of the measurementsignal 208 is determined again at a subsequent single shot EPI. Thephase difference between the single shot EPIs enables to calculate theshift of the resonance frequency. This will be explained in greaterdetail by making reference to FIG. 3.

The phase information at time TE can be obtained from the measurementsignal 208 in the time domain or in the frequency domain which requiresto perform a Fourier transformation on measurement signal 208. Therequired phase information corresponds to a central line of the k-spacewhich is usually referred to as k_(y)=0, i.e. a scan with frequencyencoding but without phase encoding. The fact that there is no phaseencoding enables to use drifts of the phase information obtained fromthe measurement signal 208 for determination of the resonance frequencyshift.

FIG. 3 shows a flow chart for monitoring and compensating of a magneticfield drift in the case of single shot EPI. In step 300 the index d isset to zero. The shift of the resonance frequency Δƒ is also set to zeroin step 300.

In step 302 the first single shot EPI is performed and the measurementof the phase φ_(a)(d) is performed as explained with respect to FIG. 2.

The time interval of TE after the first single shot EPI is performed instep 302 the second single shot EPI is performed in step 304. Again thephase φ_(a) is measured.

In step 306 a shift Δƒ of the resonance frequency is calculated bycalculating the difference between the phases φ_(a) which have beenacquired in steps 302 and 304 and by dividing the difference by 2π×thetime interval between the consecutive single shot EPIs. In this casethis time interval is the echo time TE.

In step 308 the absolute value of the shift Δƒ is compared to athreshold value. If the threshold value is surpassed a driftcompensation is performed in step 310. Preferably this is done byadjusting the frequency of the RF magnetic excitation pulses to theshifted resonance frequency.

If the shift Δƒ does not surpass the threshold no drift compensation isrequired. In step 312 the index d is incremented and the control goesback to step 304 for continuous monitoring the shift Δƒ.

FIG. 4 shows an alternative embodiment which uses a gradient echosequence rather than single shot EPI. Step 400 corresponds to step 300of FIG. 3. In step 402 the magnetic resonance data acquisition isstarted by starting to scan the k-space along a given trajectory. Thetrajectory may or may not cover the central line of the k space which isk_(y)=0.

In step 404 the scan of the k-space along the trajectory is interruptedin order to perform a data acquisition for the central k-space line withfrequency encoding but no phase encoding, i.e. k_(y)=0. This way thephase information φ_(a) is obtained.

In step 406 the scan along the k-space trajectory is continued. After acertain time interval the scan along the k-space trajectory isinterrupted again in order to perform another data acquisition fork_(y)=0 to obtain another phase information φ_(a).

In step 410 the shift of the resonance frequency is calculated bycalculating the difference between the phases determined in steps 404and 408 and by dividing the difference by 2πTE.

In step 412 the absolute value of the frequency shift Δƒ is compared toa threshold value. If the threshold value is surpassed driftcompensation is performed in step 414. In step 416 the index d isincremented and the control goes back to step 406 in order to continuethe scan along the k-space trajectory. This procedure continues duringthe entire gradient echo sequence for continuously monitoring the shiftof the resonance frequency and compensating the drift of the magneticfield if necessary.

The RF pulses of this modified gradient echo MRI sequence are applied ata single frequency (for a single slice); the phase measured at the echo(without encoding gradients) reflects only the phase differenceaccumulated between the RF pulse and the TE in case that the NMRresonant frequency (due to B0) is not equal to the excitation frequency(excluding susceptibility and chemical shift effects). When the magnetB0 is such that the NMR resonant frequency is the same as the excitationfrequency of the RF pulse then the phase error accumulated between RFpulse and TE will be zero or constant from TR to TR.

When the B0 is such that it corresponds to a different resonantfrequency than the one the RF pulse is exciting, then the phase erroraccumulated between RF pulse and TE will be a finite value (proportionalto the difference and TE). At the peak of each RF pulse, allmagnetization is in phase, irrespective of the B0. Only after this timedoes the B0 have an effect on the magnetization.

So, each RF pulse behaves like a phase reset (as far as the f0)measurement process is concerned). The relative phase error (relative tozero at the RF pulse peak) at the TE increases as B0 moves further fromthe RF excitation frequency. Other mechanisms (chemical shift,susceptibility) can contribute to the phase error measured at the TE.Assuming that these other contributors to phase error are constant fromTR to TR, the change in B0 can be determined by calculating thedifference in phase errors between data acquired form two differentTR's.

LIST OF REFERENCE NUMERALS

-   1 magnetic resonance device-   2 magnet system-   3 gradient coil-   4 supply unit-   5 transmitter coil-   6 modulator-   7 object-   9 transmission/reception circuit-   10 demodulation unit-   11 control unit-   12 processing unit-   13 monitor-   200 RF magnetic excitation pulse-   202 slice selection gradient G_(z) pulse-   204 bipolar switched gradient-   206 phase encoding gradient-   208 measurement signal

1. A method of monitoring a magnetic field drift of a magnetic resonanceimaging apparatus, the method comprising the steps of: performing afirst data acquisition by a first magnetic resonance signal being causedby a first excitation, determining a first phase of the first magneticresonance signal an echo time after the first excitation, performing asecond data acquisition by a second magnetic resonance signal a timeinterval after the first data acquisition, the second magnetic resonancesignal being caused by a second excitation, determining a second phaseof the second magnetic resonance signal the echo time after the secondexcitation, determining a shift of a resonance frequency based on adifference of the first and second phases.
 2. The method of claim 1,whereby the first and second data acquisition are performed using asignal shot EPI method.
 3. The method of claim 1, whereby the first andsecond data acquisitions are performed by means of a gradient echosequence method.
 4. The method of claim 3, whereby a k-space is scannedand second data acquisitions are performed intermittently to determinesecond phases in order to continuously monitor the shift of theresonance frequency.
 5. The method of claim 4, whereby the second dataacquisitions are performed after fixed time intervals.
 6. The method ofclaim 1, further comprising compensating the magnetic field drift bychanging the frequency of the excitation in accordance with the shift ofthe resonance frequency.
 7. The method of claim 1, further comprisingcompensating the magnetic field drift by adjusting the magnetic field.8. The method of claim 1, further comprising comparing the shift of theresonance frequency to a threshold value and compensating the magneticfield drift if the threshold value is surpassed.
 9. The method of claim1, whereby the first and second phases are determined in the timedomain.
 10. The method of claim 1, further comprising performing aFourier transformation of the first and second magnetic resonancesignals and determining the first and second phases in the frequencydomain.
 11. A computer program product, in particular digital storagemedium, for monitoring a magnetic field drift of a magnetic resonanceimaging apparatus, the computer program product comprising program meansbeing adapted to perform the steps of: determining a first phase of afirst magnetic resonance signal, an echo time after a first excitation,determining a second phase of a second magnetic resonance signal theecho time after a second excitation, whereby the second magneticresonance signal is acquired a time interval after the first magneticresonance signal, calculating a shift of a resonance frequency based ona difference of the first and second phases.
 12. The computer programproduct of claim 11, the program means being adapted to continuouslymonitor the shift of the resonance frequency.
 13. The computer programproduct of claim 11, the program means being adapted to control anexcitation synthesiser in accordance with the shift of the resonancefrequency.
 14. The computer program product of claim 11, the programmeans being adapted to control the magnetic field in accordance with theshift of the resonance frequency.
 15. A magnetic resonance imagingapparatus comprising processing means for determining a first phase of afirst magnetic resonance signal an echo time after a first excitation,for determining a second phase of a second magnetic resonance signal theecho time after a second excitation, the second magnetic resonancesignal being acquired a time interval after the first magnetic resonancesignal, and for calculating a shift of a resonance frequency based on adifference of the first and second phases and the time interval.
 16. Themagnetic resonance imaging apparatus of claim 15 having display meansfor displaying of the shift of the resonance frequency.
 17. The magneticresonance imaging apparatus of claims 15 further comprising controlmeans for controlling the generation of the excitations in accordancewith the shift of the resonance frequency.
 18. The magnetic resonanceimaging apparatus of claims 15, further comprising control means forcontrolling of the magnetic field in accordance with the shift of theresonance frequency.